1. Field of the Invention
The present invention relates to a method of manufacturing a thick-walled small diameter pipe and more particularly to a method of manufacturing a high quality thick-walled small diameter pipe which eliminates microscopic cracks which are otherwise produced on the inner circumferential surface of the pipe during pipe extension process.
2. Description of the Prior Art
A thick-walled small diameter pipe used for a fuel injection pipe of a diesel engine must have a smooth inner surface to reduce the resistance inside the pipe and to prevent the clogging at the injection nozzle. In addition, it is required that defects especially microscopic cracks on the inner circumferential surface of the pipe are reduced in order for the pipe to be able to withstand repetitive fatigue under high pressures.
This type of thick-walled small diameter pipe is made through a cold drawing process comprising repetitive pipe extension processes and heating processes performed on a pipe to be processed (original carbon steel pipe). The pipe to be processed has a black skin layer including cracks and concave puckers thereon because it is a seamless pipe and made through a cold drawing process. In this case, even if a process for removing the black skin layer is performed on the pipe to be processed using chemical means such as an acidic liquid, it is difficult to completely remove the black skin layer to modify the inner circumferential surface of the pipe to be processed into a uniform steel skin surface and the black skin layer often remains.
This can result in clogging of the nozzle due to a part of the black skin layer that peels off when such a pipe is used as a fuel injection pipe for a diesel engine.
The applicant has proposed in examined Japanese patent publication No. 51-21391 a method of manufacturing a thick-walled small diameter carbon steel pipe wherein, in forming a pipe to be processed into a thick-walled small diameter pipe, a black skin layer on the inner circumferential surface of the pipe to be processed is removed in advance (hereinafter referred to as initial internal machining) by a mechanical means (mechanical cutting or grinding) to modify the entire surface inside the pipe into a steel skin. The proposed method makes it possible to obtain a thick-walled small diameter carbon steel pipe having an inner circumferential surface which is highly smooth and excellent in circularity and uniformity and which has substantially no black skin layer on the inner surface thereof.
With the initial internal machining method according to the above-described proposal, it is possible to remove almost all the black skin layer, impurities, puckers and flaws that are left on an inner circumferential surface of a pipe to be processed which is manufactured through a hot rolling process. However, if the pipe to be processed is very uneven in thickness and is very eccentric, some of such defects may remain and, in addition, it is not possible to eliminate, during the manufacturing method for a thick-walled small diameter pipe, the defects which are newly introduced during a subsequent pipe extension process performed on the pipe to be processed.
In the manufacture of this type of thick-walled small diameter pipe, concave puckers are produced on the inner circumferential surface of a pipe to be processed during a pipe extension process conducted on the pipe to be processed after initial internal machining. The puckers then close and change to microscopic cracks. Even if the black skin layer is removed using the internal machining according to the above proposed method, cracks of sizes up to 8 .mu.m may be produced (FIG. 6 shows a crack of 74 .mu.m as an example.). If the above-described initial machining is not carried out, cracks of sizes up to 300 .mu.m may be produced (FIG. 6 shows a crack of 135 .mu.m as an example.). Further, pipes to be processed are frequently uneven in thickness and eccentric because they are manufactured through a hot rolling process Such pipes to be processed have been cut in a manner such as represented by gun drill process which is one of the methods of deep hole processing wherein a cutting tool is pushed into a pipe to be processed fixed on a jig while it is being rotated. (This process uses a tool referred to as gun drill which is aimed at obtaining the linearity of a hole, and this technique has been used in order to prevent the hole processed from becoming eccentric and curving.) Since the cutting is carried out while pushing the tool, great cutting resistance is exerted on the tool and the process can not be continued due to the deflection of an arbor caused by the pushing force unless the thickness of the arbor is increased. When a thick arbor is used to improve rigidity, the tool may advance straight without copying the inner circumferential surface of a pipe to be processed which is eccentric due to uneven thickness. In this case, a black skin layer may remain uncut even if a tool thicker than the inner diameter is used.